Scanning optical system

A scanning optical system for emitting a beam scanning in a main scanning direction, comprising:

• a light source that emits the beam;
• a line-like image forming optical system that converges the beam emitted by the light source in an auxiliary scanning direction which is perpendicular to the main scanning direction;
• a polygonal mirror that rotates about its rotational axis to deflect the beam emerged from said line-like image forming optical system in the main scanning direction;
• and
• an imaging optical system that converges the beam deflected by said polygonal mirror to form a beam spot scanning on a scan target surface in the main scanning direction,
• wherein said line-like image forming optical system forms a line-like image extending in the main scanning direction in the vicinity of a reflective surface of said polygonal mirror,
• wherein if the number of reflective surfaces of said polygonal mirror is less than or equal to six and if |m|>1.85, the following condition (1) is satisfied:
• r<5cos(w/2f)/[2|m|{1−cos(w/2f)}] (1)
• where r represents a radius of an inscribed circle of said polygonal mirror, m represents a lateral magnification of said imaging optical system in the auxiliary scanning direction, f represents a focal length of said imaging optical system in the main scanning direction, and w represents half of a scanning width.